1. Field of the Invention
This invention relates to a ball-point pen ball in which is used a nonoxide ceramic material such as silicon nitride or silicon carbide.
2. Prior Art
It is mostly the case that balls used in ball-point pens are generally in the range of 0.4-1.2 mm in diameter. If so, suppose that an average person applies a writing load of 100-250 g to the ball-point pen in which is used a ball of 0.7 mm in diameter. The contact pressure applied to the ball seat of the ball-point pen is exceedingly high, amounting to as much as 40-60 kg/cm.sup.2. Accordingly, too small a contact area of a ball adapted to be pressed into contact with a ball seat that is made of brass, stainless steel, plastics, etc. results in shaving of the surface of the ball seat by the surface of the ball.
On the other hand, the writing capability of a ball-point pen demanded from the pen during its service life amounts to a distance of about 1200-2200 m. If in this case a ball is 0.7 mm in diameter, it means that the ball revolves about one million times. Thus an important problem of wear of a ball seat involved. Accordingly, a reduction in the wear of the ball seat to the greatest possible degree involves a surface property of a ball as a very important element. Such surface property of the ball makes it necessary for ball composing materials to have the following characteristics:
(1) In order to reduce the pressure of contact of a ball with a ball seat, the component particles of ball material must be fine and uniform;
(2) In order to prevent reduction of contact pressure and shaving action, the component particles of the ball must be rounded respectively;
(3) In order to prevent reduction of the contact pressure and production of cracks, the ball component particles must be densely aggregated;
(4) In order to prevent a shaving action, the pore diameter of the porous area (hereinafter referred to as voids) produced between the component particles must be small;
(5) Recently, wherein water based ink has come to be much used in place of oil based ink, a ball component material must especially be corrosion resistant;
(6) The ball component material must have a high affinity for ink.
The above and many other rigid conditions are demanded from balls used in ball-point pens.
However, balls made of cemented carbide, hardened stainless steel, ruby, etc. in conventional use cannot satisfy all the aforestated conditions but various improvements have been made over the balls. No solution has been given to problems such as a so-called "ball sunk" (sinking of the ball into the ball seat side), "nonuniformity in writing line thickness" (a sudden supply of much ink in various points of writing line to make the thickness of the line irregular), "dripping" (trickling down of much ink at the initial time of writing), "ball break" (breaking of the ball during writing), etc.
On the other hand, a ball made of ruby is not only high in the cost of material but also takes time and labor for machining and is high in the cost of production. In addition thereto, because it is monocrystalline, such ball offers, in point of strength, the disadvantage that it tends to crack along the axis of crystal.